Obradovic believes Djokovic should talk to Federer, who after a five-year drought without a Grand Slam, won the 2017 Australian Open as well as titles in Indian Wells and Miami, rising from 16th to fourth in the world rankings within four months.

"Given that Federer was able to return into the top 10 and win this year's Australian Open at the age of 36, I see no reason why Djokovic can't be the world number one again," Obradovic said, as quoted on Reuters.

"Maybe Djokovic should talk to Federer. They are friends, they have known each other for a long time and the Swiss can certainly give him some good advice."

"All other players seem to be galvanised against him while his own motivation appears to be inconsistent and shifting all the time," he added. "He is not at his best, his serve keeps fluctuating as do elements in his game which really used to be at highest level consistently."

"However, it is difficult to play at such a high level indefinitely. Murray has a huge advantage in the standings but I am sure Djokovic will be back in contention for the top spot."